Free Height, Working Height, and Stroke on a Pogo Pin Drawing
| Drawing term | What it measures | What to check |
|---|---|---|
| Free height | Distance from the specified reference surface to the tip with no external load | Find the dimension's starting point. A through-hole solder tail may extend below the mounting surface. |
| Working height | Distance from the same reference surface to the tip at a specified compressed position | Compare the recommended height on the drawing with the actual height in the closed device. |
| Working stroke | Axial movement of the tip from its free position to its working position | Read the contact-force specification at that displacement. |
| Maximum travel or full stroke | Maximum axial displacement specified for that model | Check the manufacturer's limits for repeated operation before setting the housing stop. |
The distinction matters on low-profile parts. For example, the CPG-16-SMT-B datasheet lists a 2.50 mm working height, a 0.50 mm mid stroke, and a 0.55 mm full stroke. Only 0.05 mm separates the two listed stroke values. For a dock designed around this pin, the mounting and housing tolerances deserve a closer look than the nominal dimensions alone suggest.
How Do You Calculate Working Stroke?
Working stroke = free height − actual working height
Measure both heights along the pin axis. In the 5.00 mm example, take the pin's seating surface as the reference. Before mating, the tip sits 5.00 mm above that surface. When the dock closes, the mating pad holds the tip 4.00 mm above it. The plunger has moved 1.00 mm into the barrel.
A through-hole pin makes it easy to choose the wrong starting point. Suppose its drawing gives an overall length of 6.00 mm from the end of the solder tail to the free tip, and the tail projects 1.00 mm below the seating surface. The free height above the seat is 5.00 mm. Subtracting the 4.00 mm installed height directly from the 6.00 mm overall length would overstate compression by the full length of the tail.
If a drawing labels a dimension simply stroke, check the two positions at the ends of its dimension line. A dimension to the recommended working position and one to the fully compressed position answer different design questions.
How Do Assembly Tolerances Change the Actual Stroke?
Consider the same teaching example with two tolerances. Assume a free height of 5.00 ± 0.10 mm and a closed distance from the pin seat to the mating pad of 4.00 ± 0.10 mm. The nominal stroke is 1.00 mm. The possible working positions span a wider range.
| Assembly condition | Calculation | Design check |
|---|---|---|
| Shortest pin, largest gap | 4.90 − 4.10 = 0.80 mm | Does the pin develop enough force to maintain electrical contact here? |
| Nominal pin and gap | 5.00 − 4.00 = 1.00 mm | Does this position match the intended operating point? |
| Tallest pin, smallest gap | 5.10 − 3.90 = 1.20 mm | Is this still within the permitted working range, with enough margin for the housing to close? |
These figures illustrate tolerance arithmetic. To approve a particular pin, compare the entire 0.80–1.20 mm interval with that model's working range. On a multi-pin dock, also check the gap at each end of the row. A tilted mating surface can leave one pin lightly compressed while its neighbor approaches its travel limit.
Why Does Contact Force Specify a Compression Distance?
The spring force changes as the plunger moves, so a force value needs a position. The CPG-01-TH-B datasheet, for example, lists 80–120 gram-force (gf), typically 100 gf, at its 10.00 mm working height. The corresponding stroke is 1.10 mm. An inspector can reproduce the measurement by seating the finished pin on the specified reference and compressing it to that height.
For a magnetic charging dock, read the assembled pin's force–displacement curve at both tolerance limits. At the largest gap, check the force of the least-compressed pin and measure contact resistance at its pad. At the smallest gap, add the forces of all engaged pins and verify that the magnets can hold the dock fully seated. If the datasheet gives force at only one position, use a supplier curve or measure samples at the two actual working heights.
How Can You Verify Working Height in a Real Assembly?
Identify the surfaces that set the gap. Locate the pin seat, the face of the mating pad, and the housing stop. Measure the closed distance along the pin axis. Include the installed position of the barrel if it sits above or below the PCB surface.
Reproduce both gap limits. Use a fixture or controlled spacers to set the largest and smallest permitted gaps. Record each sample's free height, installed height, calculated stroke, contact force, and electrical result. For a charging connector, measure voltage drop at the intended current as well.
Put the acceptance conditions on the drawing. Specify free height and tolerance, permitted working heights, force limits at the relevant positions, and maximum travel as separate entries. Record the part number, drawing revision, sample quantity, and measurement reference with the results.
For a custom pin review with Dongguan Xinteng Electronics Co., Ltd., the most useful starting dimensions are the minimum and maximum distances from the pin seat to the mating pad when the housing is closed. Together with the force required at each end, those dimensions show whether a standard pin fits or whether the free height, spring, or mounting position needs adjustment.
Technical References
CPG-16-SMT-B Pogo Pin Datasheet, revision 1.0, November 4, 2024. Working height, force, mid stroke, full stroke, and mechanical drawing.
CPG-01-TH-B Pogo Pin Datasheet, revision 1.0, January 17, 2024. Through-hole mounting drawing and contact force at the specified working height.
